US President Barack Obama ignored military advice for a more modest drawdown from Afghanistan, his commanders say, suggesting his decision carried risks for the war effort.

Both General David Petraeus and Admiral Mike Mullen, the chairman of the Joint Chiefs of Staff, said Obama's plan to withdraw 33,000 surge troops by the end of next summer was more "aggressive" than they had recommended.

Asked by Senator Carl Levin on Thursday if he was prepared to resign over the war policy, Petraeus said: "I don't think it's the place for the commander to consider that kind of step unless you are in a very, very dire situation."

Petraeus, who indicated that he had received emails suggesting he should quit in protest, said: "This is an important decision, it is again a more aggressive approach than the chairman (Admiral Mullen), (Central Command chief General James) Mattis and I would have, indeed certainly, put forward.